Conjecture Concerning the Width of the Lowest Singlet–Singlet Transition in Crystalline Anthracene

Abstract

The width of the lowest singlet–singlet transition in crystalline anthracene is at least 10 times the width of (a) the corresponding free molecule transition, (b) the same transition in a diluted condensed phase. Further, in fluorescence crystalline anthracene emits lines as narrow as those in an absorption in a diluted condensed phase. In contrast, for benzene and naphthalene, the absorption lines and emission lines of the crystal, free molecule and matrix-isolated molecule have about the same widths. In this paper, we consider possible reasons for the breadth of the lowest optical absorption band of crystalline anthracene. These are (a) thermal deformation, (b) radiationless processes, (c) exciton phonon coupling, (d) exciton defect coupling, (e) pathological behavior of the exciton dispersion curve near k = 0, (f) localized absorption at the crystal surface, (g) breakdown of the selection rule k = Q near the crystal surface. We postulate that the observed linewidth arises from localized absorption by disordered molecules in the vicinity of the crystal surface.